1. Field of the Invention
This invention relates to electromagnets of the plunger type, such as solenoids, contactors, etc., and more particularly to the improvements in the sliding parts of a plunger.
2. Description of the Prior Art
Japanese Utility Model Publication No. 14036/66 discloses a coil bobbin of a plunger type electromagnet wherein the coil bobbin formed of a synthetic resinous material is provided on its inner side with a layer of a synthetic resinous material, such as Nylon, Delrin, etc., which has a low coefficient of friction and high wear resisting property with respect to steel. In this type of coil bobbin, a plunger made of metal moves in sliding contact with the resin coated inner side of the coil bobbin. Therefore, the frictional force produced between the plunger and the resin coated inner side of the coil bobbin would be great and much wear would be caused on the slidingly contacting parts, so that the electromagnet would have the disadvantage of being short in service life.
In another electromagnet of the plunger type of the prior art, it is known that the sliding characteristics and wear resisting property can be improved by using a synthetic resinous material for forming the contact surfaces of both the plunger body and the guide (in some cases the inner wall surface of the coil is substituted therefor) for guiding the plunger body in sliding contact therewith. For this purpose proposals have been made to form a ring-shaped member of polyacetal resin which is fitted over a plunger body, in producing a plunger type electromagnet.
The plunger type electromagnet constructed as aforesaid is required to have the ring-shaped resin member of a thickness of at least 0.5 to 1.0 mm, in view of the problem arising in forming the resin into the ring-shaped member. As a result, the gap between the outer circumference of the plunger body and a yoke fitted over a coil is increased and magnetic resistance is increased, thereby injuring the characteristics of the electromagnet.
Also, difficulties are encountered in selecting suitable materials for forming the ring-shaped resin member fitted over the plunger body and the guide for guiding the plunger body. In our experiences, no satisfactory results have been obtained from the tests in which various materials were tested and such materials were subjected to 5,000,000 reciprocating sliding movements, for example.